Method of purifying metals and consolidating the same



Nov. 26, 1957 w. M. HAYDEN 4 METHOD OF PURIFYING METALS ANDCONSOL'IDATING THE SAME Filed July 17, 1957 FEED OF METAL BEARING SOLIDSTREATING LIQUORS LEACH ING OPERATION WASTE TO DISCARD METAL IN SOLUTIONREDUCTION AUTOCLAVE A REDUCING GAS.

METAL POWDER INVENTORI WILLIAM M. HAYDEN W km ATTORNE s BYM ,a lag2,814,564

METHOD OF PURIFYING METALS AND CONSOLIDATHIG THE SAIVIE William M.Hayden, New York, N. Y., assignor to Chemetals Corporation, New York, N.Y., a corporation of Delaware Application July 17, 1957, Serial No.672,479

3 Claims. (Cl. 75-211) This invention relates to the production ofwrought metal articles; and more particularly it relates to recoveringcopper, nickel or cobalt in wrought form from an impure, raw materialcontaining such metal, by dissolving such metal from the raw material inan aqueous leach solution, then introducing a hydrogen reducing gas intosuch a solution at an elevated temperature and pressure to precipitatethe metal in substantially pure pulverulent form, rolling theprecipitate into consolidated bar form While in its freshly reducedstate, and then working the consolidated bar into a wrought shape.

It is well known that copper, nickel and cobalt can be recovered fromscrap or from ores or concentrations containing them by dissolving themetal from the raw material in an aqueous solution and then heating thesolution at an elevated temperature and pressure with a reducing gas.The metal is precipitated in substantially pure form, more or lesspulverulent in character. In its precipitated form the metal has at bestonly rather limited fields of use. Generally it must be converted to awrought shape in order to be useful. Such conversion, however, is not asimple matter. Melting the pulverulent precipitate and casting intoshapes for rolling is not economically desirable because such procedureentails large losses of the metal due to oxidation. Melting in vacuum orin a non-oxidizing or reducing atmosphere greatly increases the cost ofthe operation. Hot extrusion of the pulverulent metal in a reducingatmosphere has been proposed but this procedure also is costly.Fabrication of the metal into wrought or other shapes by conventionaltechniques of powder metallurgy has not proved satisfactory, partlybecause such techniques are suitable only for making rather smallarticles, and partly because the pulverulent metal precipitate has notproved itself to be uniformly suitable for fabrication by the methods ofpowder metallurgy.

I have discovered that despite the non-uniform behavior of thepulverulent precipitate in conventional powder metallurgy fabricationprocedure, it is possible to secure good and consistent results in theconsolidation of such precipitate into a bar which can be worked furtherinto a wrought shape when the precipitate is consolidated by a rollinoperation. In this operation the precipitate is passed between rollswhich preferably are arranged with their axis in a common horizontalplane. The rotating rolls work the compacted pulverulent metal mass by asort of tumbling action just prior to passage of the metal particlesinto the bite of the rolls. Then, as the metal becomes caught in thebite of the rolls, it is efiectively consolidated into bar form, inwhich form it can be handled and worked, as by sintering and furtherrolling, to produce a wrought shape.

The invention includes the discovery that when a hydrogen metalprecipitate is subjected to a rolling operation to consolidate the samepromptly after it has been precipitated or while the metal particles ofthe precipitate are still in their freshly precipitated state, or havebeen Patented Nov. 26, 195.7

maintained in such a state, a substantially superior wrought metalstructure can be produced in a rapid and continuous manner when comparedto a similar hydrogen metal precipitate which has been allowed to age toany substantial extent.

In preparing wrought metal structures it is first necessary to partiallyconsolidate the metal powder into a green compact which can besubsequently worked into the final, full density wrought metal. Thegreen compact must have sufficient green strength to withstand furtherhandling and working without breaking.

The non-aged precipitates, according to this invention, present a clean,fresh, uncontaminated metal particle surface into the bite of thecompression rolls resulting in a high green strength partiallyconsolidated compact which can be readily and easily handled andsubsequently worked in a continuous process without breaking. The highgreen strength can be obtained by a single pass through the compressionrolls.

Powders which have been aged to any substantial extent are verydifficult to bond due to the lack of fresh and clean surfaces. Partiallyconsolidated compacts formed therefrom are fragile and lack adequategreen strength for further handling or processing in a continuousprocess. Partially consolidated compacts formed from aged powdersobtained by a single pass through the compression rolls possess greenstrengths as low as one-half and below that obtainable by rolling freshor non-aged hydrogen metal precipitates under the same conditions.

Partially consolidated hydrogen metal precipitate compacts formed fromfresh or non-aged precipitates can be easily and very adequatelysintered and further worked into wrought metal shapes of a qualitysuitable for commercial use. Such partially consolidated metal compactsexhibit no blistering during the sintering operation and result inuniform, strong, full density, wrought metal structures.

The partially consolidated hydrogen metal precipitates formed from agedprecipitates blister considerably during the sintering operation and, asa result, they cannot be further Worked into wrought metal shapes of aquality suitable for commercial use. They are generally nonuniform anddo not possess adequate strength and otherwise lack good physical andelectrical properties.

The reason for the blistering of the partially consolidated compactsformed from aged powders is not known. However, it is believed to be dueto the presence of hydrogen on the surface of the precipitatedparticlesin combination with metal oxides, such as copper oxide, which forms as afilm on the precipitated particles due to aging. It is theorized thatthe hydrogen present on the metal particles reacts with the metaloxides, such as copper oxide, during the sintering operation formingwater. The water is vaporized internally of the metal compact and itsescape therefrom results in the blistering.

In addition, the partially consolidated metal compacts are generallysintered in an inert gas atmosphere such as hydrogen. The hydrogen ofthe inert gas atmosphere may also react with the copper oxide to formwater which would result in blistering. If the inert gas used containscarbon, carbon dioxide gas would probably be formed which would alsocause blistering.

If an aged precipitate is rolled any contamination on the surface of theparticles such as metal oxides is compacted internally of the partiallyconsolidated compact. The failure to remove all of these contaminantsduring the sintering operation, which causes blisters as referred toabove, would further result in a weak and non-uniform wrought metalstrip. No such problems are encountered if the hydrogen metal powder isrolled while it is still 3 fresh or maintained in the fresh stateaccording to this invention.

The problems above referred to do not exist if the metal powders are tobe melted and cast. The contaminants present are not ground into theconsolidated strip, but instead rise to the top of the melt and can beeasily removed therefrom.

Aging of the precipitate, insofar as it affects the ability of the metalparticles to be roll-bonded into a consolidated bar of good quality, isnot simply a matter of time and cannot be defined in terms of any fixedperiod of time. Much depends on how the precipitate is handled followingits formation. If it is carefully freed from leach solution and othercontaminants, and is protected from oxidation and contamination in otherways, a relatively long period of time, say a matter of weeks, mayelapse without deleterious aging of the metal particles. On the otherhand, if the precipitate is inadequately freed of leach solution orresidual salts, or if it is exposed to contaminating atmosphere, or isstored in contaminated vessels, it may age (in the sense that it losesits freshly precipitated character for purposes of roll-bonding) in amatter of hours or even minutes. The hydrogen metal precipitates can bemaintained in their freshly precipitated state by freeing them ofcontaminants such as the leach solutions, and storing them in an inertnon-oxidizing or non-contaminating atmosphere.

Prior to rolling of the metal powder, it may be, if desired, washed and/or dried. However, this is not always necessary, and the metal powdermay even be rolled directly while still wet with the liquor from whichit was precipitated, provided the powder is in the freshly precipitatedstate and is promptly subjected to the rolling operation.

The character of the metal precipitate varies considerable depending onthe particular reducing gas used to form it, and with the nature of thesolution from which it is precipitated. Precipitates from acid orsubstantially neutral solution, and precipitates formed with sulfurdioxide or carbon monoxide as the reducing gas, are coarse and grainy,whereas precipitates from alkaline (ammoniacal) solution are relativelyfine. Of all these various precipitates, I have found that those formedby the action of hydrogen on ammoniacal carbonate solutions of the metal(copper, nickel or cobalt) are most advantageous for roll-bonding. Theflow characteristics of such precipitation, the particle size and shape,and the character of the surface of the metal particles, all aresuperior to precipitates prepared from other solutions and with otherreducing gases for the purposes of working into Wrought shapes by themethod of this invention.

The process of the present invention may best be set forth in generalterms by referring to the drawing which depicts the metal powderproducing features of the present invention in the form of a flow sheet,while the metal rolling portion is depicted as an apparatus.

The impure raw material, bearing at least one of the metals selectedfrom the group consisting of copper, nickel and cobalt, is first fedinto a suitable leaching tank. A leach solution is then introduced intothe leaching tanks to effect a leaching of the raw material tosolubilize the metal values contained in the raw material. After theleaching, any waste or insoluble matter remaining is discarded. Theclear leach solution is then transferred to a reduction autoclave intowhich a reducing gas is introduced to reduce the metal contained in theleach liquor and precipitate the same in the form of a powder. Theresultant metal powder is then directly fed, after washing and/ ordrying if desired, into a hopper 2 which guides the metal powder into aroll gap formed by the pressure rolls 3 and 4. The pressure rolls 3 and4 compress the metal powder into a wrought metal form as shown at 6. Thestrip or sheet 6 is then fed into a suitable sintering furnace 7 and mayalso be subjected to further rolling operation by the rolls 8 and 9 toenhance the mechanical strength and ductility of the strip or sheet 6.

Leaching procedures to place the metal values of an ore or scrap and thelike in solution for subsequent reduction is a highly developed andwell-known art. So far as the copper, nickel and cobalt of the presentinvention are concerned, the exact leaching procedure may be widelyvaried. The most advantageous procedure involves either acid orammoniacal leaching. Either the acid or ammoniacal leach may be usedwith or without oxidation, depending upon the content of the rawmaterial being treated. Generally, sulfuric acid is used in acidleaching and some ferric sulfate is often added therewith. Ammoniacalleach liquors also often contain ammonia salts such as carbonate andcomplex metal ammonium ions. The present invention is applicable to araw material containing copper, nickel or cobalt when treated by anysuch procedures and the invention is not limited to any particular typeof leaching, the important aspect being that one or more of the metalvalues is dissolved in an aqueous solution and thus capable of beingreduced to a powdered metallic form by means of a reducing agent. Forpurposes of the following description of a specific and preferredembodiment of the invention, it will be assumed that the desired metal(copper, for example) is dissolved in an ammoniacal carbonate solution.The leach solution containing the copper or other metal values isseparated from the waste material or insoluble matter by any suitablemeans such as decanting or filtering. At this point, if desired or ifnecessary, the resultant clear solution may be purified so that itcontains but a single gas reducible metal salt. The resulting solutionis then transferred to an autoclave in which the dissolved metal isreduced to form a pure metallic powder.

The concentration of the leach liquor after or just before transfer tothe high pressure autoclave should first be adjusted so that its metalcontent is such so as to assure a good yield of the metal powder. Theleach liquor should be adjusted to a concentration of dissolved metal upto about 200 grams per liter, but below that at which in the absence ofa reducing gas a substantial amount of a compound of the metal willprecipitate when the solution is heated to above its boilingtemperature, and preferably to above 250 F. After adjustment, the leachliquor may be heated to 250 F. or higher prior to introducing it intothe autoclave, or, if desired, after it is in the autoclave. After theadjusted solution is in the autoclave, a suitable reducing gas such ascarbon monoxide or hydrogen is introduced to effect reduction of themetal salt to produce the metallic precipitate powder. The temperatureof the reduction reaction should be maintained at above about 250 F. andpreferably 300 F. to 350 F. The minimum partial pressure of the reducinggas should be about 50 p. s. i. and the total pressure should besufficient to prevent boiling. A pressure of about 900 to 1,000 p. s. i.is preferred and is suflicient to prevent boiling of the usual solution.During this reduction period the reducing gas reacts with copper (ornickel or cobalt) in the leach liquor and precipitates the metal inpowder form.

Although the maximum copper (or other metal) content is as noted above,it preferably should not exceed about l35-150 grams per liter, becauseat higher concentrations precipitation of other copper compounds islikely to occur. Generally, with ammoniacal carbonate liquors, the metalconcentration will preferably be below 130 grams per liter and sometimesadvantageously below 100 grams per liter.

Copper and other metal precipitates that have been reduced with hydrogengas produce consolidated bars that are decidedly superior toprecipitates or powders produced in other manners. Thehydrogen-precipitated copper powder, for example, yields a rolledconsolidated bar of more uniform density and greater strength in thefully rolled condition than copper powder reduced by other gases; and ityields, upon sintering and working, a final wrought shape of betterphysical properties than corresponding articles similarly prepared fromprecipitates produced by other reducing gases.

Following is a specific example, taking the production of copper powderas illustrative, of the method of the present invention:

Copper-bearing scrap was first introduced into a suit-- able leachingtank and leached by a leach liquor analyzing approximately 60-65 gramsper liter of copper (approximately 90% cupric), 150-170 grams per literof ammonia, and 100-110 grams per liter of carbon dioxide at atemperature of about 135 F. for about 6 to 8 hours. After this leachingstep the insolubles were removed from the pregnant leach solution byfiltration. The clear leach liquor was then fed into a high pressureautoclave equipped with an agitator. Hydrogen gas was introduced intothe autoclave until the pressure reached 900-1000 p. s. i. and thispressure was maintained by regulated admission of hydrogen forapproximately 35 minutes. The temperature throughout the reduction stepwas kept constant at 350 F. During the reduction step with the hydrogen,finely divided metallic copper precipitated. After the autoclave wasrelieved of its pressure, the copper powder precipitate was allowed tosettle, and then was decanted and washed. Promptly thereafter (within 24hours), while the metal particles were still in their freshlyprecipitated condition, the precipitate was fed between a pair ofpressure rolls and was thereby consolidated into bar form. Theconsolidated bar was then sintered by introducing it into a sinteringfurnace and there heating it to a temperature just under the meltingpoint of copper for a period of about two minutes. The sintered bar wasthen subjected to a further rolling operation by which itscross-sectional area was reduced about 50%. The resulting wrought copperarticle possessed excellent ductility and mechanical strength,approximating that of wrought copper.

This application is a continuation in part of my earlier applicationSerial No. 412,700, filed February 26, 1954, now abandoned.

I claim:

1. The method of recovering a metal selected from the group consistingof copper, nickel and cobalt in Wrought form from an impure raw materialcontaining such metal which comprises dissolving such metal from the rawmaterial in an aqueous leach solution, then introducing a hydrogen gasinto said solution while the solution is under super atmosphericpressure and at a temperature exceeding its normal boiling temperature,whereby the metal is precipitated in substantially pure pulverulentmetallic form, separating said precipitate from the residual solution,rolling said precipitate promptly while the metal particles thereof arestill in their freshly precipitated state into a consolidated bar, andsubjecting the consolidated bar to sintering and further rolling to forma wrought metal bar.

2. The method of recovering a metal selected from the group consistingof copper, nickel and cobalt in wrought form from an impure raw materialcontaining such metal which comprises dissolving such metal from the rawmaterial in an aqueous ammoniacal carbonate solution, heating thesolution under super atmospheric pressure to an elevated temperatureexceeding 250 F. and introducing hydrogen gas into said solution at saidelevated temperature, whereby the metal is precipitated in substantiallypure pulverulent metallic form, separating said precipitate from theresidual solution, maintaining the hydrogen metal precipitate in itsfreshly reduced state, rolling said precipitate into consolidated barform, and subjecting the consolidated bar to sintering and furtherrolling to form a wrought metal bar.

3. The method of recovering copper from an impure cuprous raw materialand working it into wrought metal form which comprises dissolving thecopper from said raw material in an aqueous ammoniacal carbonatesolution, heating said solution under super atmospheric pressure to anelevated temperature exceeding 250 F. and introducing hydrogen gas intosaid solution at said elevated temperature, to precipitate the copper insubstantially pure pulverulent metallic form, separating saidprecipitate from the residual solution, rolling said precipitatepromptly While the copper particles are still in the freshlyprecipitated condition into a consolidated bar, and subjecting said barto sintering and further rolling to form a wrought metal bar.

No references cited.

1. THE METHOD OF RECOVERING A METAL SELECTED FROM THE GROUP CONSISTINGOF COPPER, NICKEL AND COBALT INLT IN WROUGHT FORM FROM AN IMPURE RAWMATERIAL CONTAINING SUCH METAL WHICH COMPRISES DISSOLVING SUCH METALFROM THE RAW MATERIAL IN AN AQUEOUS LEACH SOLUTION, THEN INTRODUCING AHYDROGEN GAS INTO SAID SOLUTION WHILE THE SOLUTION IS UNDER SUPERATMOSPHERIC PRESSURE AND AT A TEMPERATURE EXCEEDING ITS NORMAL BOILINGTEMPEATURE WHEREBY THE METAL IS PRECIPITATED IN SUBSTANTIALY PUREPULVERULENT METALLIC FORM, SEPARATING SAID PRECIPITATEED FROM THERESIDUAL SOLUTION, ROLLING SAID PRECIPITATE PROMPT-LY WHILE THE METALPARTICLES THEREOF ARE STILL IN THEIR FRESHLY PRECIPITATED STATE INTO ACONSOLIDATED BAR, AND SUBJECTING THE CONSOLIDATED BAR TO SINTERING ANDFURTHER ROLLING TO FORM A WROUGHT METAL BAR..